In semiconductor device fabrication processes, conductive vias composed of electrically conductive materials are formed in material layers to provide interconnections for transferring electrical signals between levels of the devices. A via is fabricated by forming a through hole through the thickness of a layer. Vias are conventionally formed by photolithography and etching techniques. Vias can be formed by other techniques, such as laser drilling. A metal, such as aluminum or copper, is deposited in the via to form an electrically conductive pathway through a layer.
A problem that arises in via fabrication is achieving adequate filling of the vias with the metal. Metal is conventionally deposited in vias by sputter or electrochemical deposition. Straight via sidewalls, decreasing via diameters and increasing via aspect ratios (i.e., the ratio of via length to via diameter) have increased the difficulty of achieving adequate filling of vias with metal by sputter or electrochemical deposition.
To enhance the filling of vias with metal by sputter or electrochemical deposition, vias can be formed with sloped sidewalls (i.e., sloped vias). However, it is still desirable to form vias with vertical sidewalls. First, vias with vertical sidewalls (i.e., vertical vias) require less area than sloped vias. In submicron technologies, the extra area that is required for sloped vias reduces the maximum packing density. Second, when dry etching is used to open the vias, it is often easier to produce vertical sidewalls than sloped sidewalls. See S. Wolf and R. F. Tauber, Silicon Processing for the VLSI Era, Volume 1—Process Technology, Lattice Press, Sunset Beach, Calif., 2nd ed. 2000, pp. 770-78. Accordingly, because it is desirable to form vias with vertical sidewalls, but decreasing via diameters and increasing via aspect ratios have increased the difficulty of achieving adequate filling of vias by sputter or electrochemical deposition, it would be desirable to provide a method of forming conductive vias that can overcome disadvantages of conventional sputter and electrochemical deposition techniques.